Ideal Gas Turbine (Brayton) Plant – Assumed Nature of Compression and Expansion Under the ideal-cycle model, the compressor and turbine processes in a gas-turbine plant are treated as isentropic (adiabatic and reversible). Select the correct assumption.

Introduction / Context:
To analyze gas-turbine (Brayton) performance, we adopt an ideal model with simplified, reversible processes. These assumptions enable closed-form relations between temperatures and pressure ratio and a compact expression for thermal efficiency. This question focuses on the assumed character of compression and expansion in that ideal model.

Given Data / Assumptions:

• Simple Brayton cycle without regeneration, intercooling, or reheat.
• Negligible pressure losses in heat exchangers and passages.
• Calorically perfect gas with nearly constant gamma.

Concept / Approach:

In the ideal Brayton cycle, the compressor and turbine are modeled as adiabatic and reversible, i.e., isentropic. This yields T2/T1 = r_p^((gamma−1)/gamma) for the compressor and T3/T4 = r_p^((gamma−1)/gamma) for the turbine (with r_p the pressure ratio). These relations underpin the well-known efficiency formula eta = 1 − 1/r_p^((gamma−1)/gamma).

Step-by-Step Solution:

Verification / Alternative check:

Real machines are polytropic with efficiencies below 100%; the ideal isentropic benchmark is used to define and compute those efficiencies and to bound performance.

Why Other Options Are Wrong:

Isothermal compression/expansion is not representative for Brayton hardware; “polytropic” describes real behavior, not the ideal assumption; “isobaric” processes apply to the heater/cooler (heat addition/rejection), not the turbomachinery.

Common Pitfalls:

Applying isentropic relations at very high temperatures without accounting for variable properties; misusing absolute vs. gauge pressures when computing r_p.

Final Answer:

isentropic